Traditional analog telephones connect to the public switched telephone network (PSTN) via a two-wire analog local loop that carries both the received and transmitted audio signals. Much of the PSTN, however, operates using a four-wire path, in which separate paths are used for signals entering the network (i.e., ingress) and leaving the network (i.e., egress). At some point near the subscriber, circuitry at a telephone network facility converts the four-wire path used within the network to the bi-directional, two-wire path that connects the network to the subscriber. Circuitry within the subscriber's analog telephone converts the bi-directional, two-wire, analog loop connection back to separate, one-way signal interfaces to the handset receiver (i.e., earpiece) and from the handset transmitter (i.e., microphone). This circuit that makes this four-wire to two-wire conversion is normally referred to as a “hybrid network”.
An additional function of the hybrid network in the analog telephone is the management of the level of “sidetone” provided. Sidetone is the return of a speaker's voice from the handset transmitter to the handset receiver, and is an expected component in the signal at the handset receiver. The level of sidetone affects the loudness with which a user speaks. Too much sidetone results in the speaker talking too softly, while too little sidetone causes the speaker to talk too loudly.
An Internet protocol (IP)-based telephony network operates using separate transmit (i.e., ingress) and receive (i.e., egress) paths (i.e., a “four-wire” configuration). Because the IP telephony network has separate ingress and egress paths, IP telephones do not need to perform two-wire to four-wire conversion, and do not have a hybrid network. The absence of the hybrid network eliminates the circuitry that normally provides sidetone, prompting designers of IP station sets to create another means to provide sidetone.
Sidetone can be generated either in the software or the circuitry of the IP telephone. Much of the functionality of an IP telephone resides in a single integrated circuit referred to an “IP telephone chip”. The generation of sidetone in software is fairly straightforward. The voice samples from the handset microphone are adjusted in amplitude and mixed with the speech signals from the IP telephony network on their way to the handset receiver. In practice, however, the process places heavy demands upon the processor in the IP telephone chip, impacting overall IP telephone system performance. Moving the sidetone generation functionality into the circuitry of the IP telephone phone chip is desirable, as it reduces the cost to manufacture the IP telephone, eliminates the need for external sidetone generation circuitry, and avoids the negative effects of implementing sidetone in software.
A second use for IP telephone chips is in a device commonly referred to as a “residential gateway.” A residential gateway is a type of network gateway used to interface a packet-based IP telephony network to a conventional analog telephone loop, permitting cable, digital subscriber line (DSL), and other broadband service providers to also offer residential telephone service. In order to convert the separated egress and ingress path (four-wire) IP telephony network arrangement to the two-wire analog loop connection, an IP telephone chip in a residential gateway is typically coupled to a hybrid network. An undesirable side effect of a hybrid network is the creation of a small amount of leakage or “echo” of the signal from the egress path to the ingress path, reflecting a portion of the speech signal from the IP telephony network back to the far-end (i.e., sending) party as echo. The level of echo generated by a hybrid network is normally low, and is typically not a problem in traditional circuit-switched networks. It becomes problematic, however, on transmission paths with a large amount of end-to-end delay, most notably in networks comprising satellite circuits and packet networks (i.e., IP-based telephony networks). Circuits known as echo cancellers are normally used to minimize the audible effects of such echo. In this type of application, an echo canceller is used to remove the hybrid network leakage (i.e., echo) from the signal on the ingress path, by subtracting a scaled and delayed copy of the signal on the egress path. An echo canceller used in this application is normally referred to as a “digital hybrid balance”.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of ordinary skill in the art through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.